T-regulatory-1 (Tr1) cells are a CD4+ T cell subset with immunoregulatory function that has been identified in humans and mice. We have detected Tr1-like, enteric bacterial antigen-specific functional activity in normal mouse intestinal lamina propria (LP) CD4+ T cells, which suggests that Tr1 cells are a naturally occurring regulatory cell in the intestinal mucosa. The alms of this proposal are to test three major hypotheses: (1) that Tr1 cells are present in the intestinal LP of mice. Because there is no cell surface marker for this subset, we will use oligonucleotide microarray analysis of highly differentiated and well characterized T cell lines to identify genes or expressed sequence tags distinctive to Tr1 cells vs. other T-cell subsets using. Probes for these genes will be used to detect their presence and co-expression in LP CD4 T cells. (2) That Tr1 cells inhibit effector Th1 cells in vitro via a combination of IL-10 TGFBeta1, and cell surface CTLA4 or ICOS, mainly through effects on antigen presenting cells (APCs) and that LP CD4 Tr1 cells utilize the same mechanisms. (3) That Tr1 cells both prevent and treat the pathologic effects of Th1 effector cells in the intestine by bystander inhibition. These studies will utilize an antigen-specific model of colitis that uses DO11.10 TCR transgenic Th1 cells that are adoptively transferred into SCID recipients that are colonized with E. coli expressing ovalbumin, the antigen recognized by the transgenic Th1 cells. Collectively, these experiments will provide basic insights into how the intestinal immune system regulates the response to the enormous antigenic challenge represented by the enteric bacterial flora. Defective mucosal immune regulation to these bacterial antigens results in chronic intestinal inflammation in multiple mouse models and is postulated to occur also in patients with inflammatory bowel disease. Our long-term goal is an understanding of Tr1 cells sufficient to manipulate this subset as a novel therapy for human intestinal inflammatory diseases.